Most hydraulic system loads need variable flow for their proper operation.
Conventionally this can be achieved in three ways. One is through flow control valves which alter the flow at the expense of energy loss. Variable swash-plate axial-piston machines are frequently employed for hydrostatic drives where energy becomes a consideration. Less commonly, a fixed displacement pump can be driven by a variable speed prime-mover. The Digital Displacement™ technique, developed by Artemis Intelligent Power Limited of Edinburgh, Scotland, provides yet another way of controllably transferring energy between mechanical and fluid power.
The basic structure of a Digital Displacement machine 100, as shown in FIGS. 27 and 28, is similar to the conventional reciprocating machine, with poppet valves 102 connecting to the low and high-pressure manifolds of each cylinder. But, instead of being self-acting, each of the poppet valves 102 is equipped with an electro-magnetic actuator 104. The valves 102 are operated by a micro-controller at precise times, near the ends of the stroke, in order to establish fluid connection between the moving piston and the appropriate manifold. This control allows cylinders to behave in any of the three ways, they can pump or motor—adding or subtracting fluid from the high pressure manifold—or they can be disabled. The function of each cylinder can be changed at each end of each stroke. As the valves 102 are actuated at times in the cycle when there is almost no pressure difference across them, the actuators 104 can be compact and use little power. Either permanent magnets or springs are used to maintain the disabled poppets at default positions. A micro-controller controls the valves from its output port via a bank of power semiconductors. Digital Displacement pump-motors are described in WO 91/05163 and WO2004/025122, the entire contents of each of which are incorporated herein by way of reference. The pump-motor can be run under pressure-control, flow-control or ternary-mode of cylinder enabling.
Advantages of using Digital the Displacement technique include:
Fast response: These machines are capable of attaining either full or zero output from any starting condition, in less than a single shaft revolution.
Compatibility with micro-processor: The compatibility with micro-processors allows the use of advanced control logic. Also the same machine can be used as a pump, a motor or both.
Higher efficiency: As disabled cylinders are not pressurized, losses are reduced in comparison with swash-plate machines leading to higher efficiency, especially at part load.
Multi-banking of pump-motors: Unlike conventional machines banks of radial pump-motors can be combined along a common shaft and used as a summing junction of both torque and power whilst providing isolation between services. Accumulators may be used in conjunction with some of the banks to transfer power in or out of the system. The radial configuration provides good force balancing and gives optimal space for the mechanical components like valves and bearings.
Digital Displacement technology is ideal for building series hydraulic hybrid transmissions for automotive applications. Series hydraulic hybrid transmissions built with Digital Displacement technology can offer impressive fuel savings, packagability improvements, performance enhancement and cost savings over conventional transmissions.
This invention is a highly efficient road-going vehicle featuring an infinitely variable regenerative fluid transmission. The aim of the program is to incorporate a hydraulic automatic transmission in a standard mass-market car.
Benefits of Digital Displacement (DD) with special relevance to transmission systems include: Unprecedented part load efficiency: Cars spend little of their lives at full power, so DD's part load efficiency is particularly suited to the automotive duty cycle.
Infinitely variable transmission ratio: An infinitely variable transmission ratio allows the engine to run at its optimal RPM at all times, giving 2-liter performance from a 1.6 liter engine and improved fuel efficiency. A DD transmission makes very high overdrive ratios possible but has an almost instant kickdown to allow the engine to achieve higher speed and power during acceleration.
No dissipative clutch or torque converter: Full wheel torque is possible with the engine at tickover.
Regenerative braking with accumulator storage: Storing reclaimed energy in a hydraulic accumulator greatly improves urban cycle efficiency.
Engine off operation: The vehicle can start moving using energy stored in the accumulator without any engine power at all, allowing the engine to stop when in traffic yet retaining immediate response to accelerator pedal input.
Four-wheel independent traction control: A Digital Displacement traction control system can provide completely independent hydraulic supplies for each wheel, each controllable at high bandwidth.
Digital Displacement technology replaces the port plates and swash plates in conventional hydraulic machines with computer controlled high speed solenoid valves. The core component of a Digital Displacement system is a hydraulic piston pump/motor 100, as shown in FIGS. 27 and 28, with actively controlled poppet valves 102 which rectify the flow into, and out of, each cylinder. The cylinders are generally disposed radially around an eccentric with valving around the periphery. Banks of cylinders can be assembled along a common crankshaft to allow multiple independent outputs. The valves are each operated by a small electro-magnetic latch so that they can be opened and closed on a stroke-by-stroke basis. The solenoid coil in each latch is activated by a power FET, which is in turn connected directly to the digital output of an embedded controller.
Each cylinder has two actively controlled poppet valves, one to each of the high and low pressure manifolds. When idling (left in the diagram below) the fluid flows in and out around the low pressure valve. The high pressure valve remains closed and isolates the reciprocating cylinder from the high pressure fluid. When pumping (right), the microprocessor closes the low pressure valve to send fluid to the high pressure service.
It is also possible to hold the high pressure valve open, taking fluid from the high pressure output.
The net result of the rapid sequenced valve actuation is that, at the end of each stroke, each cylinder can be reconfigured to either pump, motor or idle. By controlling the sequence of cylinder enablings, the machine can pump fluid to a hydraulic service or accept it back (while the returning fluid actually helps to drive the crankshaft of the machine) at infinitely variable flow-rates. The valve actuation decisions are occurring every four or five milliseconds in a typical multi-cylinder pump driven at industrial diesel speeds, which gives an effective frequency response greater than 20 Hz.
The hydraulic transmission shares its generation across many pumping modules and so avoids the highly stressed line contacts inherent in gear boxes. The ability of hydraulics to relieve at a safe working pressure avoids any potential over-stressing of the driveline. Short term storage in accumulators can smooth out energy supply from sources such as wind and waves. A continuously variable transmission ratio allows slow irregular motions to be transformed into the fast, steady rotation required by generators.
In the energy storage application, Digital Displacement pump/motors can fill and empty gas accumulators to alternately store and retrieve energy. The power rating limit is in the order of MW, while the response time is in milliseconds.